Electrophotography is a useful process for printing images on a receiver medium (or “imaging substrate”), such as a piece or sheet of paper or another planar medium, plastic, glass, fabric, metal, or other objects as will be described below. In this process, an electrostatic latent image is formed on a photoreceptor by uniformly charging the photoreceptor, and then via exposure with light discharging selected areas to yield an electrostatic charge pattern corresponding to the desired image (i.e., a “latent image”).
After the latent image is formed, charged toner particles are brought into the vicinity of the photoreceptor and are attracted to the latent image to develop the latent image into a visible image. Numerous methods of development of the latent electrostatic image with charged toner particles are available. Liquid development with insulating carrier fluids including suspended charged toner particles can be used, as can processes using dry toner particles. Common dry toning processes include both mono-component and two-component toning systems. Mono-component toning systems generally apply dry toner particles to a development roller by way of a foam roller, a doctor blade, or both; the development roller then presents the charged toner to the electrostatic latent image on the photoreceptor. Two-component toning systems usually include toner particles and oppositely charged magnetic carrier particles, the mixture of which is called a two-component developer. The two-component developer is attracted to a magnetic brush toning apparatus, which then supplies the developer to the latent electrostatic image. Note that the visible image might not be easily visible to the naked eye depending on the composition of the toner particles. The practice of the present invention is described in terms of dry toner processes, but is not confined to such.
Control of the quantity of toner deposited on the final receiver is critical to the proper performance of the electrophotographic printing device. A typical process control system utilizes a method of sensing the amount of toner deposited, and reacts to the result of such a measurement by controlling imaging process parameters to keep the amount of toner at a desired optimal level. Although there are many methods available to accomplish the sensing of the amount of toner deposited, the present disclosure relates to an optical sensing method operating at infrared light wavelengths. The amount of toner deposited is referred to as toner coverage, developed mass per unit area (DMA), and image density, among others. These terms are taken to be synonymous. DMA is usually specified in units of milligrams per square centimeter, or mg/cm2.
As used herein, “toner particles” are particles of one or more material(s) that are transferred by an electrophotographic (EP) printer to a receiver to produce a desired effect or structure (e.g., a print image, texture, pattern, or coating) on the receiver. Toner particles can be ground from larger solids, or chemically prepared (e.g., aggregated from a dispersion of a pigment and latex resin particles, or prepared from an organic phase comprising toner ingredients and a solvent suspended in an aqueous phase followed by removal of the organic solvent), as is known in the art. Toner particles can have a range of diameters, for example, less than 8 on the order of 10-15 or up to approximately 30 Diameter refers to the volume-weighted median diameter, as determined by a device such as a Coulter Multisizer. Toner is also referred to in the art as marking particles, dry ink, or developer in the case of mono-component toning sub-systems.
Toner includes toner particles, and can also include other particles. Any of the particles in toner can be of various types and have various properties. Such properties can include absorption of incident electromagnetic radiation (e.g., particles containing colorants such as dyes or pigments), absorption of moisture or gasses (e.g., desiccants or getters), suppression of bacterial growth (e.g., biocides), adhesion to the receiver (e.g., binders), electrical conductivity or low magnetic reluctance (e.g., metal particles), electrical resistivity, texture, gloss, magnetic remanence, fluorescence, resistance to etchants, and other properties of additives known in the art. Toner particles themselves can be coated with even finer particles known as surface treatment agents. Such fine particles can be sub-micron to a few microns in size, and are added to enhance properties such as the free flow ability of the bulk toner powder, the toner triboelectric charging characteristics, and the toner transfer efficiency. Surface treatment agents in common use include pyrogenic silica, colloidal silica, titania, alumina, and fine resin particles, among others. The surface treatment agents themselves are commonly coated with compounds including a wide variety of types of silanes and silicones.
Toner particles can be substantially spherical or non-spherical. The shape of toner can have a large influence on its performance in the electrophotographic process, and factors in the toner manufacturing process can be used to control the shape but can also introduce unintentional shape variability. For example, the shape of toner can affect electrostatic transfer efficiency, bulk powder flow properties which affect behavior in the toner replenisher hopper, and bulk powder flow properties of two-component developers. The latter can affect the amount of developer fed to the toning roller and thus the resulting image DMA and optical density. Toner particle shape also has an effect on the scattering of light, including at infrared wavelengths. Highly shaped toners reflect or scatter more light than less shaped toners at a given DMA; spherical toners scatter the least light. Toner particle shape particularly affects the ability of a mono-component toning subsystem to provide a smooth layer on the development roller through action of the foam application roller and the doctor or metering blade. In general, smoother shapes perform better, with the result of a trade-off of lower sensitivity of the DMA sensor. Toner particle shape can be variable according to natural but unwanted variation in toner manufacturing processes. Thus, there is the need to provide a DMA sensing system that provides improved robustness to variations in toner shape.
The most common toner particles are solid in that they contain resins, colorants, additives and the like, but not voids which contain air. Toner particles can however be porous in that they can contain voids, vesicles, pores, cavities or inclusions of air. The voids can be discrete or interconnecting. The words voided, vesiculated, porous, foamed and expanded are taken to be synonymous.
After the latent image is developed into a toner image on the photoreceptor, a suitable receiver is brought into juxtaposition with the toner image. A suitable electric field is applied to transfer the toner particles to the receiver (e.g., a piece of paper) to form the desired print image. The imaging process is typically repeated many times with reusable photoreceptors. The photoreceptor is typically in the form of a drum or a roller, but can also be in the form of a belt. In some configurations, the toner image is first transferred to an intermediate transfer member, from which the visible image is further transferred to the final receiver. Thermal transfer processes are also useful in the same manner.
The receiver is then removed from its operative association with the photoreceptor or intermediate transfer member and subjected to heat or pressure to permanently fix (i.e., “fuse”) the print image to the receiver. A plurality of print images (e.g., of separations of different colors) can be overlaid on one receiver before fusing to form a multi-color print image on the receiver.
The electrophotographic printing process as just described is characterized by plural sub-systems that influence the amount of toner transferred to the final receiver; these sub-systems can change their behavior over time or in response to conditions experienced by the printing process. Process control sub-systems are commonly employed that can manipulate the operating parameters of such variable imaging subsystems to maintain the amount of toner transferred to the ultimate receiver at a desired level. For example, in dry two-component development sub-systems, the concentration of toner in the developer mixture comprising toner particles and magnetic carrier particles (% TC), influences the amount of toner developed. Usually higher % TC leads to higher toner developed mass per unit area (DMA) due to factors including an increase in the rate of development and a decrease in the charge per mass of the toner itself. Magnetic toner concentration monitors are used to measure % TC and thus enable control of the rate of addition of replenisher toner to the developer in order to keep % TC at the desired value as toner is removed at variable rates due to variable coverage product images. Process control sub-systems can also control the rate of replenishment of toner and thus % TC through knowledge of the coverage amounts specified in the digital files to be printed. Other process control strategies let % TC vary according to the signal from a sensor that records the DMA at some point in the process such as on the photoreceptor after toning, or on an intermediate transfer member, or on the output images themselves. For example, if the operating environment becomes less humid and causes an increase in charge per mass of the toner and a resulting decrease in DMA, the signal from the DMA sensor is responded to by increasing the rate of replenishment and thus increasing % TC in order to bring the DMA back up to the desired value. This is a simple and common process control scheme, that depends on the sensitivity and robustness of the DMA sensing method to produce optimal output image quality and stability. Thus, there is a need for sensitive and robust developed mass per unit area sensing as described by the present invention. Note that a developed mass per unit area (DMA) sensor can also be referred to as an image density control (IDC) sensor or a toner coverage sensor.
Photoreceptors typically do not maintain stable discharge response to light over time or use. The degree to which they can be discharged, the surface potential to exposure contrast, and the efficiency at which they can be charged are subject to change. Process control sub-systems may include surface potential sensors which the control system responds to by selecting charging voltages and exposure levels in order to help maintain DMA at a desired level. Such changes may likely require changes to control parameters in the toning sub-system such as the toning roller bias voltage level in order to maintain the desired DMA and toner background level of an output print. Changes in the gain of the development sub-system lead to DMA changing in an unwanted manner. Examples include % TC variability, changes in humidity and temperature that result in toner charge per mass changes, developer aging processes and lot-to-lot variability in toner tribocharging properties, among others. In order to overcome these and other electrophotographic process instabilities, modern process control methods usually employ DMA sensing feeding back to photoreceptor charging voltages and exposure levels in combination with changes to development sub-system parameters in order to keep DMA and the resulting image density at the desired levels. There is thus a need for highly sensitive and robust methods of measuring the developed mass per unit area of toner. There is also a need to keep the cost of the DMA sensing subsystem to as low as possible in order for printers and equipment manufacturers to thrive in today's competitive business climate.
A common architecture of a color printer includes four imaging modules, one for each of the cyan, magenta, yellow and black primary colors, operating together continuously in a parallel process. Each imaging module includes the necessary sub-system components including the photoreceptor, charging means, exposure means, development means, cleaning means, etc. Such imaging modules are arranged around an intermediate transfer belt, to which the four color toner images are sequentially transferred in register from the four photoreceptors. In this manner, the complete image is formed on the transfer belt, from which a final transfer is made to the ultimate receiver such as a sheet of paper. The transfer belt commonly has static dissipative properties; the necessary level of resistance is usually achieved through the use of carbon black as a conductive filler. Thus, most intermediate transfer belts are opaque and black in color. Thus, the developed mass per area (DMA) sensor typically cannot be a transmission densitometer measuring the absorbance of color process control patches through the intermediate transfer belt. A typical sensor used to measure the DMA of cyan, magenta, and yellow process control patches as transferred to the intermediate web instead measures the amount of light scattered by the toner in such patches at infrared wavelengths of light. Thus, there is a need for sensitive and robust sensing to measure scattered light at infrared wavelengths. The black toner presents a special case, as carbon black is typically used as the black colorant and carbon black is a strong absorber in the infrared. The black toner DMA sensor in such printers is based on the absorbed infrared light from a process control patch on the belt. The geometry of the sensor is such that the detector is arranged to collect light reflected from the glossy surfaced intermediate belt, which is then modulated by the DMA of the black toner.
U.S. Pat. No. 5,410,388 to Pacer et al. describes a process control scheme to compensate for toner concentration drift of a two-component development system due to developer aging effects. A sensor configured to measure reflectance is used to detect lead and trail edge densities of large process control patches on a web-based photoreceptor, which are responded to by controlling parameters such as toner concentration, development bias voltage and photoreceptor potential to keep image quality constant. The sensor is based on a semiconductor light emitting diode with a 940 nm peak wavelength and a 60 nm one-half power bandwidth. The use of an infrared wavelength reflective sensor detecting toner patches on a photoreceptor is thus illustrated. U.S. Pat. No. 5,436,705 to Raj provides another example of TAC (toner area coverage) measurement on the photoreceptor using an infrared reflectance sensor. Both references refer to black and white processes.
U.S. Pat. No. 5,991,558 to Emi et al. describes the use of a reflective sensor, operating with light at infrared wavelengths, where there is a single emitter and two detectors. One detector is oriented to the base medium at the equivalent angle to that of the emitter, such that specularly reflected light from the base medium is detected when there is no toner on the medium. If the base medium is a typical black intermediate transfer belt, the presence of a patch of carbon-black-based black toner provides a lower signal; thus, a measure of the coverage of the black toner is characterized. For color toners without carbon black, a greater signal-to-noise is realized when the detector is oriented to collect only light that is diffused, which is greater when toner is present than not. In this manner, the coverage of color toners such as cyan, magenta and yellow are measured. The sensor operates in the infrared, at 970 nm.
FIG. 1, adapted from U.S. Pat. No. 5,991,558, illustrates the positions of the emitter and the two detectors, one oriented at the equivalent angle to that of the emitter to collect light specularly reflected from the media, and one mounted at an angle to collect scattered or diffused light. Herein the words scattered light and diffused light are used interchangeably. A toner coverage sensor 31 (i.e., a DMA sensor) is placed in opposition to the process element 1 (e.g., media) where toner images will be located to be sensed. The toner coverage sensor 31 includes an infrared emitter element 32 (e.g., an LED) which illuminates the process element 1 at an illumination angle α. A specular radiation detector 34 is oriented to detect light which is specularly reflected from the process element 1. A diffused radiation detector 36 is oriented to detect diffuse light which is scattered by toner particles on the surface of the process element 1. The patent discloses selective use of one or the other of the detectors depending on the coverage of the test patch of toner to optimize the detected signal. FIG. 2, also adapted from U.S. Pat. No. 5,991,558, illustrates the sensor output 46 for color toner using the diffused radiation detector 36, and the sensor output 44 for black toner using the specular radiation detector 34. A data processing system (not shown) can be used to determine the toner coverage from the sensor output using calibration functions determined by characterizing the sensor output as a function of toner coverage. Developed mass per unit area sensors with this orientation scheme of one emitter and two detectors are in common use in the electrophotographic printer industry.
A graph 50 showing the absorbance of light as a function of wavelength for a representative commercially available set of cyan, magenta, yellow and black toners is illustrated in FIG. 3. These spectral absorbance functions were measured for toner samples removed from the C504S, M504S, Y504S and K504S cartridges used in a Samsung Xpress C1810W printer were electrostatically coated onto a clear film support at a coverage of approximately 0.4 mg/cm2, fused in a roller fuser apparatus to leave a smooth, uniform and continuous layer of toner, and measured for optical absorbance in transmission as a function of wavelength from 350 nm to 1050 nm on a Perkin-Elmer UV-VIS model Lambda 35 spectrophotometer. It can be seen that for the cyan, magenta and yellow colorants used in these toners there is essentially no absorption of light above 850 nm in the infrared region of the spectrum. In an exemplary configuration, the color toners absorb less than 5% of the radiation in the infrared wavelength band sensed by the toner coverage sensor 31 as measured by the method used in FIG. 3, where a toner deposit of 0.4 mg/cm2 fused to a G60 gloss of at least 20 on clear support is measured for optical absorbance in transmission at the wavelengths in the infrared wavelength band of the toner coverage sensor 31.
The Samsung C1810W printer uses a toner coverage sensing system with the geometry illustrated in FIG. 1, sensing the toner coverage on a smooth (shiny) black intermediate transfer element. The light from the emitter element 32 of the toner coverage sensor 31 was measured to be centered at approximately 930 nm. Thus, to measure toner coverage (i.e., the DMA) of the primary cyan, magenta and yellow color toners, the process control sensor detects the reflection of 930 nm infrared light with the diffused radiation detector 36. The greater the amount of toner per unit area, the greater the amount of light that is scattered, thus the greater the signal detected by the diffused radiation detector 36. On the other hand, for the black toner, where carbon black is the primary colorant, light is absorbed at significant levels from 850-1050 nm. Thus, infrared light will be largely absorbed rather than scattered and detection is accomplished using the specular radiation detector 34 where the presence of black toner on the smooth black colored intermediate transfer element will lower the amount of specularly reflected light.
Many pigments and dyes have been used as the colorants in commercially available cyan, magenta, and yellow toners. The large majority do not significantly absorb light at wavelengths from about 850-1050 nm, and are thus optimally detected using a diffused radiation detector 36 operating in this range of infrared wavelengths.
U.S. Pat. No. 5,625,857 to Shimada et al. describes a deposited toner amount sensor where the light receiving element has a wide light-receiving area to receive at least a part of irregularly reflected (scattered) light besides specularly reflected light. The use of such a complex sensor illustrates the need to improve the sensing of cyan, magenta and yellow toners. The advantage of using infrared light is described in column 6, lines 41-48, where it is noted that doing so thus reduces effects caused by differences of color toners.
U.S. Pat. No. 9,020,380 to Shida describes toner coverage sensing using devices operating at 950 nm, with geometries including a single emitter and two detectors arranged so as to separately collect specularly reflected light and scattered light. FIG. 3 of U.S. Pat. No. 9,020,380 describes a sensor of geometry essentially identical to that of FIG. 1 discussed earlier. FIG. 1 of U.S. Pat. No. 9,020,380 shows an embodiment where such a sensor is set up to measure patches of unfused toner transferred to an intermediate transfer belt element. It is stated that “in order to detect a test pattern with a sensor, the test pattern must be made larger than the spot diameter of the light irradiated by the sensor. On the other hand, the developer consumed in density control is considered wasted consumption on the part of the apparatus by the user, and must be reduced as much as possible” (col. 1, lines 44 to 49). Thus, the need for improved process control sensing where a minimal amount of toner in a test patch can yield a larger signal-to-noise is desirable.
U.S. Pat. No. 3,879,314 to Gunning et al. discloses a process for making porous polyester granules designed for use in paints. The authors state that “if vesiculated polymer granules in which the vesicles are vapor-filled are incorporated in a paint composition, they can, unlike extender pigments used hitherto as flatting agents in paint, contribute opacity to a dry film of the paint by reason of their vesiculated structure” (col. 1, lines 25 to 30). The particle making process includes preparing an aqueous dispersion of a pigment, dispersing this fluid as droplets in an unsaturated polyester dissolved in a polymerizable monomer, dispersing the resulting mixture as droplets in water containing dispersing and thickening components, followed by polymerization. The pores or vesicles result after drying of the droplets of the internal water phase containing the pigment. This is known in the art as the “double emulsion” method. The granules described are however too large for use as a modern toner.
U.S. Pat. No. 3,923,704 and U.S. Pat. No. 4,137,380, both to Gunning et al., disclose improved processes for making porous polyester granules designed for use in paints. The granules are of particular use as opacifying matting agents in latex paints and avoid the defect observed hitherto of cracking at high film builds. Formulation improvements over that of the U.S. Pat. No. 3,879,314 reference are described.
U.S. Pat. No. 4,461,849 and U.S. Pat. No. 4,489,174, both to Karickhoff, describe improved processes of manufacturing vesiculated beads which have special utility as opacifying agents for paints and show improved scattering efficiency and resistance to shrinkage upon drying. As with the prior three references just described, a water-in-oil-in-water emulsion, or double emulsion method, is used. The vesiculated beads are about 0.1 to 500 microns in diameter; vesicle diameters range from about 0.01 to 5.0 microns, preferably from 0.03 to about 1.0 micron.
U.S. Pat. No. 7,572,846 to Engelbrecht et. al. describes improved vesiculated particles for use in paints. The particle preparations described are variants of the double emulsion polymerization method. The use of cross-linking and suitable hydrophobic monomers is described such that the particles are left with a hydrophobic surface that is said to hinder the re-entry and re-adsorption of water when the cross-linked particles are dry. Improved opacity, whiteness, scrub resistance and water resistance of paints are said to be realized.
U.S. Pat. No. 5,409,776 to Someya et. al. discloses a multi-shell emulsion particle of dry state structure having one or more penetrating pores connecting the surface layer of the particle with the interior of the particle. The particles are prepared by emulsion polymerizing a mixture of monomers including 5% to 80% of an unsaturated carboxylic acid to form particles which are then added to a second emulsion polymerization step with vinyl monomers at a specified ratio with the first emulsion particles, followed by treating the resultant multi-shell emulsion polymer with an alkaline material to neutralize and swell the polymer. A third polymerization step is optional after the neutralization or swelling step. The emulsion particles are said to offer improved hiding power and brightness as an organic pigment.
U.S. Pat. No. 5,608,017 to Kamiyama et. al. discloses a suspension polymerization method for producing polymerized particles having cavities in the particles. The method described is essentially a double emulsion method where the monomer(s) to be suspension polymerized are suspended at the desired droplet size in water, where the monomer droplets themselves also contain dispersed droplets of an incompatible liquid such as water. The cavities are created by drying the polymerized particles. The particles are said to be useful as space retention agents, lubricity providing agents, functional carriers, standardization particles, toners, functional fillers, and the like. The reference does not discuss the light scattering properties of such cavity containing particles.
U.S. Pat. No. 7,741,378 to Cui describes polymerization methods to prepare spherical, monodisperse porous acrylic particles. Monodisperse polymethylmethacrylate seed particles are swollen with oil-soluble polymerization initiators, monomers including methyl methacrylate and divinylbenzene to 20 to 80 times the mass of the original seed particles, followed by polymerizing the monomers. The porous monodispersed particles that result are said to be usable as a carrier that can incorporate a variety of pigments, pharmaceutical agents, and the like, and are suitable for use as various types of adsorbents, columns, and the like, because of their porosity. Moreover, the colored monodispersed particles according to the invention are monodispersed and spherical, while containing a large amount of pigment. The colored monodispersed particles are thus described as being usable as a display element of electronic paper, a spacer for liquid crystal display panels, a toner for printers, a cosmetic product, and the like. The reference does not discuss the light scattering properties of such particles.
U.S. Pat. No. 4,254,201 to Sawai et. al. discloses a toner capable of being fixed by pressure alone rather than being fixed by fusing at high temperatures. The toner consists of porous aggregates or clusters of individual granules of a pressure-sensitive adhesive substance, each granule being encapsulated by a coating film of a film-forming material. The toner is prepared by granulating spray dried particles. The porosity is important to the ability of the toner to be pressure fixed. The reference does not discuss the light scattering properties of such toner particles.
U.S. Pat. No. 4,379,825 to Mitushashi discloses a porous electrophotographic toner and a process to prepare such a toner. The toner is prepared by mixing and kneading under heat ingredients including coloring matter, a binder, and an elimination compound, pulverizing the resultant mixture, and removing the elimination compound by treating the powder with a solvent. The elimination compound must be chosen to be of the desired pore size, and so as to not melt during the high temperature kneading step. Examples given of the elimination compounds include dyestuffs which can be removed with an organic solvent which is not a solvent for the binder, and sodium chloride, sodium carbonate or saccharose starch which can be removed by water where water is also not a solvent for the binder. The advantage of the toner is said to be its ability to be pressure fixed under low pressure. The reference does not discuss the light scattering properties of such a toner particle.
U.S. Pat. No. 7,368,212 to Sugiura et. al. describes porous toner particles with a specified degree of porosity, size of pores, and toner circularity. The particles are prepared by dispersing in water a solvent containing the necessary components to form toner including a prepolymer which is then reacted to become elongated or cross-linked and components that undergo a degassing process to liberate a gas such as carbon dioxide which causes the pores to be formed. The advantage of the toner is said to be the ability to the lower the developed mass per unit area, called toner adhesion in the reference, while maintaining good required properties such as chargeability, transferability and fusibility. The authors do not discuss the light scattering properties of such toner particles.
U.S. Patent Application Publication 2013/0011782 to Sano et. al. discloses polymer-expanded particles, methods to prepare polymer-expanded particles, and expanded toner prepared by such a method. The preparation includes mixing and impregnating toner with high pressure gas or supercritical fluid, followed by reducing pressure and temperature to expand the toner material (generate porosity), which is then crushed and classified to the desired toner particle size. The authors do not discuss the light scattering properties of such toner particles.
U.S. Pat. No. 9,005,867 to Mang et. al. discloses a process to prepare porous toner particles by a variant of the emulsion aggregation toner method. Emulsion aggregation toner is prepared by controlled aggregation of an aqueous emulsion of resin particles, pigment particles and other optional toner addenda such as wax particles. The authors show how washing the filter cake from a slurry of emulsion aggregation toner particles with an alcohol results in porous toner particles. Advantages of porous toner particles are said to include requiring less toner mass to accomplish similar imaging results, thus lowering cost per page, providing a thinner image to reduce curl and image relief, saving fusing energy and providing a look and feel similar to offset printing (see col. 2, lines 37-53). The authors do not discuss the light scattering properties of such toner particles.
Commonly-assigned U.S. Pat. No. 4,833,060 to Nair et. al., which is incorporated herein by reference, describes the preparation of toner or polymer particles by a technique called evaporative limited coalescence (ELC). Toner ingredients such as the binder resin, colorants, waxes and charge control agents are dissolved or dispersed in a water immiscible solvent such as ethyl acetate, forming an oil-phase. This solution is then sheared into an aqueous mixture including a surface-active promoter polymer and colloidal silica as a particulate stabilizer to form oil-phase droplets the size of which are controlled by the amount of colloidal silica added. The pH of the aqueous phase can be controlled by a buffer. The solvent is then evaporated to form solid toner or polymer particles. After the shearing step, the colloidal silica functions to limit the coalescence of oil-phase droplets into larger droplets when the surface concentration of the silica on the droplets becomes approximately a monolayer. Thus, using more silica results in greater particle surface area, and thus smaller droplets and smaller resulting solid particles after solvent removal. The evaporative limited coalescence method as described produces resin particles or toner particles that have a very narrow distribution of particle sizes, which are solid without porosity. The colloidal silica can be removed by treatment in an alkaline aqueous solution, and the particles can be washed of aqueous phase salts. Further additives such as flow aids can be applied to the surface of the toner as needed. The shape of such particles can be varied by adding a shape control agent which tends to bind together the colloidal silica on the surface of the oil-phase droplets such that more surface area of the final solid particle results after the evaporation step to remove the solvent. Commonly-assigned U.S. Pat. No. 6,207,338 to Ezenyilimba et. al., U.S. Pat. No. 6,380,297 to Zion et. al., and U.S. Pat. No. 6,482,562 to Ezenyilimba et. al., each of which are incorporated herein by reference, describe preferred embodiments of shape control methods which can be used to prepare toner using the evaporative limited coalescence process. Shapes can range from spheroidal to highly folded and oblong. The shaped toners described by these references are solid without porosity.
Commonly-assigned U.S. Pat. No. 7,754,409 to Nair et. al., which is incorporated herein by reference, describes a method of manufacturing porous toner particles including: providing a first emulsion of a first aqueous phase comprising a pore stabilizing hydrocolloid dispersed in an organic solution containing a polymer; dispersing the first emulsion in a second aqueous phase; and evaporating the organic solution from the droplets to form porous toner particles of a controlled size and size distribution. This is commonly known as the evaporative limited coalescence process when a particulate material such as colloidal silica is used to stabilize the oil in water emulsion. The pores are created by the presence of the hydrocolloid stabilizer contained in the first aqueous phase, which is dispersed in the organic solution phase. Toner ingredients such as pigments, waxes and charge control agents can by dissolved or dispersed in the organic solution. A second double emulsion process is described where the organic phase comprises polymerizable monomers resulting in porous particles after polymerization. The disclosure states that “there is a need to reduce the amount of toner applied to a substrate in the electrophotographic process. Porous toner particles in the electrophotographic process can potentially reduce the toner mass in the image area. Simplistically, a toner particle with 50% porosity should require only half as much mass to accomplish the same imaging results. Hence, toner particles having elevated porosity will lower the cost per page and decrease the stack height of the print as well. The application of porous toners provides a practical approach to reduce the cost per print and improve the print quality” (see col. 2). The authors do not discuss the light scattering properties of such toner particles.
Commonly-assigned U.S. Pat. No. 7,867,679 to Nair et. al., which is incorporated herein by reference, describes porous toner particles prepared by a variant of the evaporative limited coalescence technique previously described. Two solvents are used in the oil-phase, where the second less volatile organic solvent is a poor solvent for the binder resin. Non-ionic organic polymer particles are added to stabilize pores which are created when the solvents are evaporated. The advantage of such porous toner is said to be a reduction in the toner mass in the image area, which will reduce toner cost per printed page. The thinner image is said to improve image quality, reduce curl, reduce image relief, save fusing energy and offer a look and feel closer to offset printing. The authors do not discuss the light scattering properties of such toner particles.
Commonly-assigned U.S. Pat. No. 7,887,984 to Nair et. al., which is incorporated herein by reference, describes porous toner particles prepared by a variant of the evaporative limited coalescence technique previously described. A preferred embodiment uses a double emulsion method where a first aqueous-phase with a dissolved hydrocolloid such as carboxy methyl cellulose resin is dispersed in an oil-phase containing dissolved or dispersed toner ingredients such as resins, pigments, waxes and charge control agents. The oil-phase solvent is immiscible in water such that the oil-phase which contains droplets of the first aqueous phase can itself be dispersed as droplets within a second aqueous phase comprising a particulate stabilizer such as colloidal silica. After evaporation of the solvent and water, pores are formed from the first aqueous phase droplets within the oil-phase containing the necessary toner ingredients. The particles have a porosity of at least 10%. The advantage of such porous toner particles is said to be a reduction in the amount of toner applied to the substrate by an electrophotographic process. Porosity can lower toner stack height, lower cost, and improve print quality. The authors do not discuss the light scattering properties of such toner particles.
Commonly-assigned U.S. Pat. No. 8,252,414 to Putnam et. al., which is incorporated herein by reference, describes porous particles and porous toner where an additive such as a pigment or wax needed for a toner composition can be incorporated into the pores (also known as microvoids). The particle preparative methods are variants of the evaporative limited coalescence process described in previously cited references. The advantage of such porous toner is said to be a reduction in the mass of toner in the image area, resulting in lower cost per page, lower toner stack height, and improved image quality. The authors do not discuss the light scattering properties of such toner particles.
Commonly-assigned U.S. Pat. No. 9,029,431 to Nair et. al., which is incorporated herein by reference, describes porous particles made by variants of the evaporative limited coalescence double emulsion method where a hydrocolloid is used to stabilize the cavities. The ability to vary the shape of such particles is discussed in column 13. Such porous particles are said to be useful for chromatographic columns, ion exchange and adsorption resins, drug delivery devices, cosmetic formulations, papers and paints. Previous patents that describe the use of such particles as toner are mentioned. However, the authors do not discuss the light scattering properties of such toner particles.
Commonly-assigned U.S. Pat. No. 9,376,540 to Boris et. al., which is incorporated herein by reference, describes porous polymer particles prepared by the evaporative limited coalescence double emulsion process that have discrete pores of different pore sizes stabilized by different hydrocolloids. The authors state that “Porous polymeric particles of controlled size are useful in diverse applications such as physical spacers, gaseous absorbers, optical barrier and diffusers, permeable barriers, electrophotographic toners, lubricants, desiccants and dispersive media. Porous polymeric particles having discrete pores of controlled size are likewise of technological importance to these and other applications where precise control of particle density, optical scatter, particle modulus, or elasticity or internal porous surface area is advantageous.” However, the scattering properties of porous color toner particles are not further mentioned or detailed.
Commonly-assigned U.S. Patent Application Publication 2012/0077000 to Putnam et al., which is incorporated herein by reference, describes voided or porous toner particles prepared by a chemical method. An improved image fusing process is realized with the combination of specified fuser topcoat properties and toner with pores or voids. It is shown that, compared with solid toner, porous toner results in reduced relief of the toner image, reduced lateral spread of the image during fusing, and reduced fusing conditions. However, the scattering properties of voided color toner particles are not mentioned. It should be noted that porous toner particles collapse to solid films during the toner fusing process.
There remains a need for improved toner coverage sensing systems for electrophotographic printers that provide a higher measurement sensitivity compared to prior art configurations.